Telephone units which can operate in a speakerphone mode are popular and widely available both for landline and radio communication. Such units (‘speakerphone units’) include a microphone and a speaker (loudspeaker) arranged to receive and transmit speech information in a hands-free operation. Speakerphone units may be used as mobile telephone units in vehicles. Speakerphone units typically suffer from certain drawbacks, which often make their use difficult or inconvenient. In particular, speakerphone units may be highly susceptible to interference from local noise. In particular, decreasing size and cost of manufacture of the units leads to decreasing acoustic isolation between the output audio device such as a speaker or earpiece and the microphone of the units. The output of the speaker/earpiece is simultaneously recorded by the microphone and then undesirably re-transmitted, hence creating a direct-path echo which interferes with speech of the spoken words.
In addition, when an external speaker and microphone are in use in a mobile environment in a vehicle, reverberations of the output acoustic signal inside the vehicle constitute indirect echos. To overcome the problem of such external echos, it is known in the prior art to synthesise a replica of the echos by modelling the echo path and then subtracting the result from the transmitted signal. An algorithm commonly used for this so called ‘echo-cancellation’ comprises a LMS (least mean square) based procedure, for low cost implementation. Unfortunately, the cancellation of the residual echo by such procedures is not absolute.
When establishing a call between two telephone units, namely unit A and unit B, e.g. mobile radio units in vehicles, an echo coupling will occur in unit A when unit B receives a speech signal, or is picking up background noise (for instance unit B is in a noisy car). In this situation, an audio signal will be produced at the speaker of unit A and part of it will be returned to the microphone of unit A as a residual echo. If unit A does not receive any significant locally generated audio signal at its microphone, this type of echo is known as ‘far-end only’. In the situation where the user of unit A produces speech which is picked up by the microphone of unit A and there is no signal at the speaker of unit A (no echo exists) this is known as ‘near-end only’. Whenever in unit A there is a signal from unit B and the user of unit A is talking at the same time there will be at the microphone of unit A a mixed signal of echo and ‘near-end’. This is known as ‘double-talk’.
In use of LMS based echo-cancellation algorithms an echo-suppressor algorithm is sometimes also employed because the amount of attenuation achieved by echo-cancellation is insufficient. This is true in various communication conditions but the returned echo is particularly perceived in ‘far-end only’ echo conditions as described above. Therefore, the echo-suppressor is usually arranged to operate a switching device to provide disconnection of the microphone if a certain level of residual echo noise is present after operation of the echo cancellation algorithm. The level is selected by assuming the lowest attenuation of the returned echo signal during typical operation and it is normally based on echos which comprise speech signals which are difficult for proper processing due to their non-stationary characteristics. The echo suppressor decisions which determine whether the switching device is to be operated are usually made by determining a ratio or difference between the measured input energy from the microphone and the measured output energy being sent to the speaker, and comparing this ratio or difference to a pre-determined fixed threshold. Echo suppressors of this kind cannot distinguish between energy at the loudspeaker due to speech signals and noise.
Therefore, if radio A communicates with radio B placed in noisy environment (for example a noisy car, possibly using a vehicle adaptor for the radio) the background noise received from radio B might easily cause the echo suppression system in radio A to switch off the microphone because the received signal has a high noise content. Thus, the microphone signal of radio A will be frequently cut off and communication becomes very difficult or impossible. The microphone cutting off is known as microphone ‘clipping.’
In order to overcome this frequent clipping problem the signal from the microphone may be replaced with a substitute signal or the microphone may be diconnected for a long period. Neither of these solutions is satisfactory.
An example of a prior art arrangement including an echo cancellation portion and an echo suppression portion is described in WO00/35603A the contents of which are incorporated herein by reference.
One purpose of the present ivention is to provide improved communication between units wherein one or both of the units is in a noisy environment, especially by having less frequent microphone clipping in communication between units in which microphone clipping by an echo suppression system is provided.